I. Field of the Invention
The present invention is directed to an improvement in vehicle-mounted police radar warning receivers, and more particularly to such a receiver which can distinguish between pulsed or continuous signals from a fixed frequency source (e.g., police radar) and such signals from a variable frequency source (e.g., non-police radar).
II. Description of the Prior Art
Police radar operates in the X-band and K-band of the frequency spectrum as discussed in U.S. Pat. No. 4,313,216, assigned to Cincinnati Microwave, Inc., the assignee herein. There are, generally, two types of police radar. One emits a continuous radar signal so long as the radar unit is turned on. The other emits a brief burst of radar signal when the police officer triggers the unit. This latter type is referred to as pulsed or instant-on radar. While transmitting, both continuous and pulsed radar transmit a signal which is at a fixed frequency within the selected band.
An electronic assembly referred to as a police radar warning receiver has been devised to detect the presence of police radar signals. The receiver is mountable in a vehicle, such as a passenger car or truck, motorcycle, boat or the like, which travels on land or water in areas subject to speed-monitoring radar surveillance by police, and functions to detect the presence of the police radar and provide the driver or user with an audible and/or visual indication that his speed is being checked by radar. The receiver is contained in a box-like housing which is set on the dash or clipped to the visor in the vehicle. Extending from the rear of the housing is a power cord which terminates in a plug adapted to be received in the cigarette lighter socket of the vehicle. The front panel of the receiver faces the driver and has various indicators and control knobs.
When police radar is operating within range of the radar warning receiver, the circuitry of the receiver is able to detect the presence thereof. The ESCORT and PASSPORT radar warning receivers, manufactured by the assignee herein, Cincinnati Microwave, Inc. of Cincinnati, Ohio, utilize a superheterodyne circuit for this purpose. As explained in aforementioned U.S. Pat. Nos. 4,313,216, and in U.S. Pat. No. 4,581,769, which is also assigned to the assignee herein, a superheterodyne circuit employs two local oscillators, one of which sweeps in frequency over a range of frequencies related to one or both radar bands. A first oscillator signal is mixed with the incoming police radar or other signal to produce a first IF signal. The first IF signal is then mixed with the other oscillator signal to produce a second IF signal which is then passed through a discriminator circuit to provide output pulses if a signal is present. As is understood, such a heterodyning process will result in generation of a "duplicate" or image of the police radar signal within the receiver. Hence, the discriminator generates a pulse related to the actual frequency signal received as well as a subsequent pulse related to the image frequency of the received signal.
One advantage of a superheterodyne circuit is that neither oscillator need operate at a frequency within the range of actual radar frequencies. Thus, filters can be used in the antenna to reduce leakage out of the receiver of energy generated by the oscillators without also filtering out the received radar signal. Further, any oscillator energy which might leak out of the receiver will not appear as a police radar signal to another radar warning receiver as it will be outside the range of frequencies of interest to that other receiver.
Some radar warning receivers have been introduced to the marketplace using the so-called superhomodyne scheme for detecting frequencies in the X- and K-bands. These receivers use a signal generator or local oscillator operated at or near the same frequency as the signal to be received, and this internal signal tends to be re-emitted by the antenna of the receiver. Although its power level is low, the proximity of a receiver of this type to a sensitive receiver could make it appear that a police radar transmitter is in the vicinity, thus sounding an alarm. Since the local oscillator in a superhomodyne receiver is at or near the same frequency as the received signal, it is impossible to trap that signal and thus prevent it from being re-radiated by the antenna without also interfering with reception of police radar signals.
As a result, receivers of the superhomodyne type are continuously broadcasting X- and K-band signals which must be discriminated against by other receivers so as not to erroneously indicate presence of a police radar signal. As discussed in aforementioned U.S. Pat. No. 4,581,769, the signals emitted from superhomodyne receivers have one characteristic, however, that permits their signals to be distinguished from police radar signals, and that is the frequency of the superhomodyne emitted signal is constantly varying over the range of frequencies that it is designed to detect and thus, when discriminated, will generate what appear to be random pulses from sweep to sweep. Taking advantage of that characteristic, the circuitry disclosed in the afore-mentioned U.S. Pat. No. 4,581,769 utilizes a "pulse position" technique which relies upon the time relative position of each pulse generated by the discriminator in each sweep of the local oscillator. For a fixed frequency signal, e.g., police radar, the primary and image pulses will be generated at the same time in the sweep from sweep to sweep. The pulses resulting from a spurious homodyne receiver signal will appear to move from sweep to sweep. Hence, unless the pulses are in the same location during consecutive sweeps, the receiver can ignore the incoming signal as it is not likely a police radar signal. Relying upon the apparent time compression affect of receiving such a spurious signal which causes the discriminator output to appear to contain high frequency components, other receivers merely low pass filter the discriminator output pulses thereby preventing them from reaching alarm enabling circuitry.
Various other electronic products designed to operate in the radar bands have been proposed for use with automobiles and the like. Recently, for example, a radar-based system has appeared which may be utilized to reduce the likelihood of collisions. One such system transmits a radar signal in the X-band from one vehicle and receives reflections thereof from a second vehicle to control the braking system of the first vehicle or to warn the driver thereof that one vehicle is approaching the other at too great a speed. This collision system operates in the X-band. Hence, the signal generated thereby appears to a radar warning receiver to be a police radar signal. An alarm would, therefore, be generated. Indeed, neither the "pulse position" nor the low pass filter techniques discussed above are believed capable of discriminating against this new spurious signal so as not to unnecessarily generate an alarm.
The "pulse position" technique is not believed to work because the signal emitted by the collision system does not vary across the band as do spurious homodyne receiver signals. Rather, the collision system emits a signal which is in the nature of a frequency-shift-keyed (FSK) signal. The FSK signal is centered at about 24 GHz with a 400 KHz separation. That is, the signal emitted by the collision system will toggle back and forth between 23.9998 GHz and 24.0002 GHz. Because the separation is so close, the signal as received by the radar warning receiver would appear as though it were essentially at 24 GHz, i.e., from a fixed frequency source. Even were the radar warning receiver sensitive enough to receive and discriminate both the lower and higher frequency signals as two separate signals, the toggle rate of the collision system is so fast (about 16.4 KHz) that the radar warning receiver would detect what would appear as two signals at the respective fixed frequencies during each sweep. Hence, with the "pulse position" technique, there would merely be more repeated pulses in the same location from sweep to sweep and the receiver would still generate an alarm indicating reception of a police radar signal.
Similarly, the low pass filter technique is not believed to be a workable solution, either. When a radar signal is received, the output of the discriminator has a characteristic S curve with a fundamental frequency related to the sweep rate of the local oscillator and the bandwidth of the discriminator. Receipt of a spurious homodyne receiver signal causes the discriminator output to "time-compress", i.e., the S curve will appear as though the sweep rate of the local oscillator had increased. As a result, the discriminator output will have an effective fundamental frequency much greater than the fundamental frequency as in the case of the received radar signal. Thus, a low pass filter with a cutoff above the fundamental frequency (radar signal) and below the apparent fundamental frequency (spurious homodyne receiver signal) will effectively filter out the spurious homodyne receiver signal. However, the collision system signal will generate an S curve output from the discriminator with essentially the same fundamental frequency as that generated by receipt of a radar signal but with added frequency components due to the 16 KHz toggle rate. Use of a low pass filter to eliminate such a signal would also interfere with proper operation of the police radar warning receiver. That is, the cut-off frequency of the filter would have to be so low that it would preclude proper detection of true police radar signals as well.